This invention relates to a procedure for acquiring orientation of a spacecraft relative to the earth during an encirclement of the earth by the spacecraft and, more particularly, in a first embodiment of the invention, to a procedure employing an alignment of a yaw axis of the spacecraft with the sun followed by a scanning movement of the spacecraft for sighting the earth by an earth sensor.
The orientation of a satellite, or spacecraft, encircling the earth is described in terms of a local coordinate system centered at the spacecraft and having three mutually perpendicular axes, namely, a yaw or z axis, a roll or x axis, and a pitch or y axis. In the case of a geosynchronous spacecraft traveling along an essentially circular orbit around the earth, and correctly oriented with the earth, the positive z axis points toward the center of the earth, and nominally, without yaw biasing, the positive x axis points in the direction of travel along the flight path of the spacecraft. The x, y and z axes form a right handed coordinate system.
In the performance of many types of missions, it is essential for the spacecraft to maintain its orientation relative to the earth during travel along a path encircling the earth. An example of such a mission is the generation of a sequence of photographs of the earth's cloud cover, wherein a displacement of certain cloud features among successive ones of the photographs would be indicative of cloud movement. The accuracy with which the cloud movement can be determined is dependent on the stability of the spacecraft orientation because any instability in the orientation would give a false reading of cloud displacement among the sequence of photographs. A further example of a mission requiring stable orientation arises in the case of a communications satellite wherein antenna radiation patterns are directed to specific geographical areas. A rotation of the spacecraft away from its desired orientation would offset an antenna radiation pattern from its designated geographical area resulting in degradation of the communication.
In the event that the spacecraft orientation becomes destabilized, it is important to reestablish the desired orientation rapidly. This is readily appreciated in the case of the communications mission wherein a lapse of several hours for reestablishing spacecraft orientation would create an unacceptable inconvenience to persons utilizing the communications function of the spacecraft. A problem arises in that with presently available procedures, the amount of time required to reestablish orientation is excessively long and that, furthermore, implementation of the procedures may require a significant amount of aid from a ground station which tracks the spacecraft. Preferably, the reorientation of the spacecraft should be accomplished with little or no aid from a ground tracking station.